The present invention relates to a speaker system that detects the oscillation of the voice coil unit with a high degree of accuracy, thus enabling a high fidelity reproduction by the speaker system.
For some time, a detection unit that detects the oscillation displacement of an oscillation system that comprises a voice coil unit, an oscillation plate, and a dust cap has been installed in speaker systems. The detection results by the detection unit are employed in generating a negative feedback through acquiring the difference between the detected oscillation displacement and the input signal. Thus, by compensating the error of the oscillation system (the difference between the detected oscillation and the input signal), a high fidelity reproduction can be achieved. A speaker system that comprises a speaker drive circuit which adjusts the input signal by compensating the error detected by a detection unit is described in Japanese Patent Publication Number 2940587, which is incorporated herein by reference.
With regard to the detection means, Japanese Laid-Open Utility Model Application Publication (Kokai) Number S 63-16789 (FIG. 1) discloses a technology in which the displacement of the oscillation system is detected by affixing a reflecting member to the inner surface of the dust cap, positioning an optical sensor facing the reflecting member, and optically detecting the displacement of the dust cap.
Another example of a detection means is disclosed in Japanese Patent Publication Number 2940587 (paragraph 35, FIG. 1, etc.). In this reference, the displacement of the oscillation system is detected by affixing the reflecting plate not to the inner surface of the dust cap but to the lower edge portion of the voice coil unit, arranging a light sensor directly below the reflecting member, and optically detecting the displacement of the voice coil unit.
However, there are problems associated with affixing the reflecting plate to the inner surface of the dust cap, as disclosed in the first patent reference above. For example, during reproduction, mechanical flexion and sympathetic vibrations are produced in the dust cap. These effects degrade the degree of accuracy of detecting the displacement of the oscillation system.
There are also problems in a system where, as disclosed in the second patent reference above, the reflecting plate is affixed to the lower edge portion of the voice coil unit.
One problem is that the reflecting plate causes a weight distribution imbalance of the voice coil unit with respect to a center axis of the voice coil. The original oscillation action of the voice coil unit without a weight imbalance is a piston motion, in which the voice coil unit oscillates in parallel to the center axis of the voice coil unit. With an imbalance, a rolling oscillation that involves an inclination from the center axis of the voice coil unit is produced. Therefore, the rolling oscillation caused by the weight imbalance degrades the accuracy of the detection of the original oscillation action of the voice coil unit. In addition, because of the rolling oscillation, ideal oscillation is not possible and sound quality becomes relatively degraded as a result.
A second problem is that the voice coil is a source of heat generation. Accordingly, it is necessary to construct the reflecting member, which is attached to the voice coil unit, from heat resistant materials. The construction of the reflecting member with a metal material, which has superior temperature resistant properties, is one option. However, since metal materials are relatively heavy, the use of metal for the reflecting member increases the weight of the voice coil unit. This added weight adversely affects the original oscillation action of the voice coil unit.
The reflecting member may also be constructed from a light-weight resin material, rather than a metal material. The use of a light-weight resin material minimizes the increase in the weight of the voice coil unit. However, when heat resistance properties are taken into consideration, the resin materials that can be used are limited to resins with superior heat resistant properties (for example, engineering plastic). The resins with superior heat resistant properties are relatively high in cost, which increases the manufacturing cost of the overall system.
A third problem is that, because of the limitations in the size of the system, the light sensor is disposed directly below the reflecting member and must be arranged in the vicinity of the voice coil unit. This creates a problem because the characteristics of the light sensor may change or, depending on the circumstances, the light sensor may be destroyed due to the heat generated by the voice coil.
Embodiments of the present invention address the problems discussed above and, in particular, provide a speaker system with which the oscillation of the voice coil unit is detected with a high degree of accuracy and with which high fidelity reproductions are possible.